PROBLEM DESCRIPTION
Two disk write errors on disk blocks. The format/read throws no defect on the surface. I would like to know how to determine if the write error was due to a disk defect or to another source. Morover, I would like to know how to early detect future write errors on disk.

Basically there is no easy way to determine a disk failure. In your situation 2 retryable errors on the same block on the same disk should not be of any concearn to you. Format -> Analyze -> Read ... and the issue ir resolved.
Also, there is no threshold on the errors that would indicate a failure. It's more of a common sense in reviewing error counters in iostat considering uptime etc. Also: